1. Field of the Invention
This invention relates to an optical disc recording apparatus wherein base band signals are recorded on an optical disc having a recording medium having a larger thermal time constant by a pulse width light modulated recording light.
2. Description of the Prior Art
A compact disc in which digital audio signals are recorded on a reproduce-only optical disc is now in widespread use. An optical disc has also been evolved in which the recording layer is formed by an optical recording medium making use of the optical or magneto-optical signal recording and reproducing principle. As an optical disc compatible with respect to the compact disc, a write-once optical disc (CD-WO) on which the information can be written only once or an overwritable magneto-optical disc (CD-MO) on which data can be rewritten is also finding practical usage and application.
The recording layer of the write-once optical disc (CD-WO) is formed by an optical recording medium, the state of which is changed by heat produced by the irradiated recording light, such as, for example, an organic dyestuff type optical recording medium.
With the optical disc recording/reproducing apparatus for recording or reading data on or from a write-once disc or an overwritable disc, as the optical disc is rotationally driven at a constant angular speed or a constant linear speed, the recording track of the optical disc is scanned by a light beam emitted by an optical head in which there are enclosed a laser diode driven by a laser driving circuit for outputting the laser light for recording and/or reproducing the information, a photodetector for detecting the irradiating laser light reflected by the optical disc, and so forth, for recording and/or reproducing the digital data.
It is known that, when the data are recorded on the optical disc of the optical disc recording and/or reproducing apparatus, the error rate depends in general on the power of the laser light used for writing the information, such that it is necessary to write the information at the optimum laser power.
When recording the CD signals on the optical discs, such as CD-WO or CD-MO, it is also necessary to produce the playback output consistent with the CD standards for assuring compatibility of the CD-WO or CD-MO with respect to the compact disc.
However, as illustrated in FIG. 1, when the baseband signals are recorded on an optical disc the recording layer of which has been formed by an optical recording medium, the state of which may be changed by heat produced by the irradiated recording light which has been light pulse width modulated by the baseband signals, a recording delay may be caused as a result of thermal time constants of the optical recording medium, with the result that the pit length on the recording layer becomes longer than the regular length corresponding to the baseband signals. That is, when the recording light having a width corresponding to the pulse width of the baseband signal is irradiated on the disc, with the baseband signal being the CD signal represented by EFM data (eight/fourteen modulated data) having pulse widths of from 3T to 11T, where T indicates a clock bit duration, a time delay .tau..sub.B which elapses between the end of irradiation of the recording light and the end of the formation of a pit P.sub.A is longer than a time delay .tau..sub.F which elapses between the stated of irradiation and the start of formation of the pit on the recording layer. The result is that the length L.sub.PA of the pit P.sub.A actually formed on the recording layer becomes longer than the length L.sub.PO of a regular pit P.sub.O corresponding to the pulse width of the baseband signals. Above all, with an optical disc having its recording layer formed by an optical recording medium having a larger than normal thermal time constant, the recording pit pattern is drastically affected by the recording delay due to thermal time constants of the optical recording medium.
Hence, with the CD-WO optical disc in which CD signals represented by the EFM data with pulse widths of from 3T to 11T are recorded on a recording medium of a larger than normal thermal time constant, the central LS of the eye pattern of the playback output is not in a correct matching state as shown in FIG. 2, while a modulation factor of 60% or more cannot be achieved with the preservation of the asymmetry in accordance with the CD standards unless recording is made at an excess power. On the other hand, a recording layer formed by an organic dyestuff base optical recording medium has a larger time constant and tends to be increased in push-pull on pit formation, such that, should recording be made at an excess power, it becomes extremely difficult to maintain the push-pull (mean value) according to the CD standards within the range of from 0.04 to 0.07.